1. Field of the Invention
The present invention relates to a semiconductor light source lighting circuit that lights a semiconductor light source such as a light emitting diode (LED) or the like.
2. Description of Related Art
In recent years, vehicular lamps, such as headlamps and the like, use LEDs that are long in service life and low in power consumption, instead of conventional halogen lamps that have filaments. The degree of light emission of an LED, that is, the brightness thereof, depends on the magnitude of electric current that flows through the LED. Therefore, when an LED is to be used as a light source, it is necessary to provide a lighting circuit for adjusting the current that flows through the LED. Such a lighting circuit usually has an error amplifier and performs a feedback control such that the current that flows through the LED becomes constant.
It is desirable that the brightness of an LED be adjustable because, for example, there are a high-beam state and a low-beam state of a headlamp, and also in order to make it easy to meet standard requirements. Available methods for changing the brightness of an LED include a method in which the value of electric current is continuously changed and a pulse width modulation (PWM) light dimming method in which current is switched on and off to change the duty ratio. In the former method, a color shift in which the color tone or the color temperature changes depending on the value of current can occur. Therefore, the LED lighting circuits of vehicular lamps often adopt the PWM light dimming method.
In Japanese Patent Application Publication No. 2010-170704 (JP 2010-170704 A), assigned to the same assignee as the assignee of the present application, a lighting control apparatus that adopts the PWM light dimming method has been proposed.
In the lighting control apparatus described in JP 2010-170704 A, the value of LED current detected during a period in which a switching regulator is driven is retained in an analog manner by using capacitors during a stop period that follows the elapse of that driving period. However, generally, the capacitors provided for maintaining the value of current in an analog manner have relatively large capacity. Therefore, there is a possibility of the size of the circuit becoming unnecessarily or inconveniently large